1. Field of the Invention
The present invention relates to a diaphragm valve having a suck-back function.
2. Description of Related Art
In semiconductor manufacturing processes, conventionally, a predetermined amount of chemical solution are applied to semiconductor wafers to coat each wafer surface with a thin film. Recently, a micro fabrication technique in the semiconductor manufacturing processes has been promoted. Under the circumstances, need arises to exactly control the amount of chemical solution to be applied in order to form the thin film of even thickness. In a semiconductor manufacturing device, correspondingly, a diaphragm valve provided with a suck-back function has been proposed to prevent dripping of the chemical solution from a nozzle tip when supply of the chemical solution is stopped. FIGS. 5 and 6 are schematic sectional views of a diaphragm valve disclosed in Japanese patent unexamined publication No. 2003-185053. In particular, FIG. 5 illustrates the diaphragm valve in an open state and FIG. 6 illustrate the same in a closed state.
A diaphragm valve 100 has a valve part 101 which is partitioned by a partition wall 111 into an upper and lower chambers. In the valve part 101, a piston 112 is centrally placed penetrating the partition wall 111 and normally urged downward by a spring 113. The piston 112 is fixed, at its bottom end, with a diaphragm 115 made of fluorocarbon resin (e.g., Teflon®). The diaphragm 115 is a film member whose outer edge portion is fixed to an inner wall of the valve part 101 and central portion is fixed to a bottom surface of the piston 112.
A valve seat 116 is formed in the valve part 101 at a center of a bottom. A valve chamber 132 in the valve part 101 is connected in fluid communication with a suck-back chamber 134 in an auxiliary part 102 through a connecting pipe 133. In the auxiliary part 102, a diaphragm 117 identical in shape to the diaphragm 115 is placed with an outer edge portion fixed to an inner wall of the auxiliary part 102. These diaphragms 115 and 117, vertically aligned and placed in the same orientation as shown in FIGS. 5 and 6, are connected by a connecting rod 130 mounted coaxially with a center axis Q of the valve 100.
The diaphragm valve 100 constructed as above is operated as follows. When air is supplied into the valve part 101 (i.e., the upper chamber) through an air pipe 140, the piston 112 is pushed up against the resilient force of the spring 113 as shown in FIG. 5, bringing the diaphragm 115 out of contact with the valve seat 116 while deforming the diaphragm 115. Simultaneously, the diaphragm 117 coupled to the diaphragm 115 by the connecting rod 130 exhibits similar motion to that of the diaphragm 115. In this way, the diaphragm valve 100 is opened to allow a liquid entering in the valve chamber 132 through an inlet passage 131 connected to a tank not shown storing a chemical solution to flow through the connecting pipe 133 and the suck-back chamber 134 and to discharge the liquid through an outlet passage 135 connected to a nozzle for discharging the chemical solution. When a leak valve not shown is opened, on the other hand, the piston 112 is moved downward, pushing out the air from the upper chamber through the air pipe 140, by the resilient force of the spring 113 until the diaphragm 115 is brought into contact with the valve seat 116. Thus, the diaphragm valve 100 is closed. At this time, the diaphragm 117 exhibits just the same motion as that of the diaphragm 115.
In diaphragm valves, generally, the motion of a diaphragm causes changes in volume on a downstream side of the diaphragm, which may lead to inappropriate draw-back and push-out of the liquid at valve opening and closing times respectively. In the diaphragm valve 100, however, which is provided with the diaphragm 117 identical in shape to the diaphragm 115 in the suck-back chamber 134, the volumes of the valve chamber 132 and the suck-back chamber 134 are changed exactly equally, but increased and decreased inversely with each other, by the motions of the diaphragms 115 and 117 respectively during the opening/closing operation of the valve 100. Accordingly, the volume change of the valve chamber 132 caused by the motion of the diaphragm 115 is absorbed (counterbalanced) by the motion of the diaphragm 117, so that the inappropriate draw-back and push-out of the liquid can be prevented.
However, the conventional diaphragm valve 100 shown in FIGS. 5 and 6 has a problem that it could perform the suck-back function in theory but not sufficiently in practice because the diaphragm 117 constructed as shown tends to be distorted or warped by the fluid pressure.
This is because that, in the diaphragm valve 100, the diaphragms 115 and 117 are arranged in a vertically aligned relation and in the same orientation; specifically, as shown in FIG. 6, the diaphragm 115 is placed having an outwardly curved portion with respect to the valve chamber 132, whereas the diaphragm 117 is placed having an inwardly curved portion with respect to the suck-back chamber 134. When the liquid is allowed to flow from the inlet passage 131 to the outlet passage 135, the fluid pressure exerts on the diaphragm 115 forming a part of the valve chamber 132 and the diaphragm 117 forming a part of the suck-back chamber 134 respectively. Accordingly, the diaphragm 115 receives the fluid pressure, from below, on the under surface of the outwardly curved portion, so that the diaphragm 115 will hardly be distorted or warped by the fluid pressure. On the other hand, the diaphragm 117 receives the fluid pressure, from above, on the upper surface of the inwardly curved portion. The diaphragm 117 is therefore likely to be crushed and dented.
If the diaphragms 115 and 117 formed with a small thickness for flexibility are irreversibly deformed by the fluid pressure, they may undergo a distortion which impairs the opening/closing operation of the valve 100 or affects the suck-back function. Furthermore, the irreversibly deformed diaphragm 117 would be damaged in a short time, resulting in a short life, which needs frequent replacement, leading to an increase in cost.
In addition to the above problems, the diaphragm valve 100 further has a disadvantage in structure. Specifically, the pipes forming the inlet passage 131, the connecting pipe 133, and the outlet passage 135 are connected to the box-shaped housings forming the valve part 101 and the auxiliary part 102. It is normally conceivable that those components are assembled through welding connection; however, such assembling is unpractical due to difficult replacement of the short-life diaphragm 117 as mentioned above. In the structure shown in FIGS. 5 and 6, furthermore, it is hard to replace the diaphragm 117 placed with the outer edge portion fixed to the auxiliary part 102.